In the automotive industry, fuel tanks of been increasingly made of polymers, especially polyethylene. These plastic fuel tanks are lighter and cheaper than the traditional ones made of sheet steel. Aside from this, they can be formed, without difficulty, into complex shapes so that the available space can be better utilized and the tank's volume can be increased.
Plastic fuel tanks are, however, not fully impermeable to fuel since small amounts of fuel constantly escape to the outside by permeation. There is, recently, a demand to substantially reduce the rate of permeation. This can be attained with good success by exposing the inner surface of the tank, for a period of time, to the influence of a fluorine containing treatment gas. The surface is, thereby coated with fluorine in the form of fluoridated carbon compounds and fluoridated hydrocarbons. This fluorine containing layer considerably reduces the rate of permeation. If one simultaneously uses the fluorine containing treatment gas as a blow medium during the blow extrusion of the fuel tank, one is referring to the so-called in-line-fluoridation. If, on the other hand, one applies the fluoridation to fully extruded fuel tanks, one refers to off-line-fluoridation. Such processes are, for example, known from German DE-PS 24 01 948 and DE-PS 26 44 508.
However, the fluoridation of plastic surfaces influences not only the permeation to a large degree, but also influences the abrasion resistance, the chemical, thermal and mechanical stability, the adhesion properties and the wettability. For this reason, the invention is not restricted to the inner surfaces of fuel tanks. The surface of the polymer is subjected to the attack of elementary fluorine during fluoridation. In the simplest case, as for example, polyethylene, the result is a stepwise radical substitution of the CH-- bonds by CF-- bonds.
As our own extensive experiments have shown, this results in fluoridated layers which are structured quite differently depending upon the conditions of the reaction. In order to obtain specific advantageous and uniquely reproducible surface effects for the above mentioned material behavior, it is of considerable importance that one adheres exactly to specific structure parameters of such a fluoridated surface. It is a question here, in the first place, of layer thickness, homogeneity of the fluorine coating, distribution of CH.sub.2 --, CHF-- and CF.sub.2 -- groups, and of the depth profile. Aside from this, the variable surface reactivity of many plastics, which varies from charge to charge must be considered. In order for specific fluoridated layers, selection and adherence to definite reaction conditions is of paramount importance. There is, however, no suitable process available for this, one makes do with empirical values obtained by experience.